Fixing SIWAREX WP231 IO Access Errors on S7-1200 PLC Systems

1. Problem Overview

Engineers integrating the SIWAREX WP231 electronic weighing module into a SIMATIC S7-1200 PLC frequently encounter a red LC (Load Cell) status LED combined with the diagnostic buffer entries "New IO access error during process image update" on the CPU and "Downloaded and existing configuration are different" on the technology module. The PLC enters the diagnostic state immediately after the hardware configuration is downloaded, even when no user program has been written yet.

The root cause is almost always physical: the load cell is either unconnected, miswired, or wired with the wrong polarity on the 7-pin terminal block of the WP231. A second, less common cause is a firmware/checksum mismatch inside the module that is not cleared by re-downloading TIA Portal hardware configuration. This article documents the diagnostic path, the load cell pinout, the TIA Portal configuration verification steps, and the checksum recovery procedure published in the SIMATIC ET 200SP / SIWAREX documentation set.

2. Affected Hardware and Configuration Context

The WP231 is mounted to the left of the S7-1200 CPU and occupies a single signal-module slot. The module exposes three terminal blocks: a 7-pin block for the load cell, a 4-pin block for the RS-485 interface, and a 2-pin block for 24 V power. Diagnostic and configuration data are exchanged through the cyclic process image of the S7-1200 backplane.

3. Root Cause Analysis

Two distinct faults are reported in field reports:

1. "New IO access error during process image update" in the S7-1200 diagnostic buffer — the CPU cannot read/write the WP231 I/O because the module has not completed startup. The CPU logs an I/O access error and the SF (System Fault) LED on the CPU lights red.
2. "Downloaded and existing configuration are different" in the WP231 diagnostic buffer — the module has detected a discrepancy between the configuration stored in its non-volatile memory and the configuration currently being downloaded from TIA Portal, or the module has not been able to read a valid weighing record at all.

When the LC LED on the front of the module is solid red, the weighing ASIC has not detected a stable bridge voltage. This is the dominant cause on commissioning day and resolves itself once the load cell is correctly wired to the 7-pin terminal block.

4. SIWAREX WP231 Load Cell Pinout

The 7-pin load cell terminal on the WP231 is laid out as follows, viewed from the wiring side of the detachable plug:

*Wire colors vary by load cell manufacturer. Always cross-check against the datasheet of the actual cell.

Critical wiring rules:

• Use a shielded, twisted-pair cable (e.g., LiYCY 4×2×0.34 mm²) between the load cell junction box and the WP231.
• Ground the shield at the WP231 end only; leave the cell end floating to avoid ground loops.
• For a 6-wire load cell, connect the sense leads (SEN+, SEN−) to the WP231 sense terminals. Sense lines compensate for voltage drop in long excitation cables and are required for installations where the excitation cable run exceeds approximately 5 m.
• For a 4-wire load cell (no sense leads), short SEN+ to EXC+ and SEN− to EXC− at the WP231 terminal block using short jumper wires on the plug itself. This forces the internal excitation monitor to see the actual bridge voltage.
• Maximum cable length from WP231 to load cell: 500 m with 6-wire connection; 100 m with 4-wire connection.

5. SIWAREX WP231 Status LED Reference

6. Step-by-Step Diagnostic Procedure

Follow this sequence before editing the TIA Portal project:

1. Power-cycle the S7-1200 station with the WP231 connected but with the load cell cable disconnected from the module. Note the LC LED state. If LC stays green, the module is healthy and the fault is on the load cell side.
2. Measure the excitation voltage between EXC+ (pin 7) and EXC− (pin 6) with a multimeter. A healthy reading is 9.5 V to 10.5 V DC. Reading of 0 V or < 8 V indicates a backplane power fault or a damaged module.
3. Reconnect the load cell following the pinout in Section 4. Torque the terminal screws to 0.5 Nm to 0.6 Nm.
4. Measure the bridge output between SIG+ (pin 3) and SIG− (pin 2) with a calibrated millivolt meter. With no load applied, the reading is the no-load zero offset; with a known calibration weight applied, the reading should change proportionally. Typical span: 0 to 10 mV for a 2 mV/V cell at full load, with 10 V excitation.
5. Verify the shield termination. The shield must be bonded at the WP231 end only, using the grounding clamp provided on the module's bottom face. A shield tied at both ends creates a ground loop and injects 50/60 Hz common-mode noise that will surface as unstable weight readings and intermittent diagnostic events.
6. Power-cycle again and observe the LC LED. When wiring is correct, the LC LED transitions to green within 3 to 5 seconds of the module completing startup.

7. Configuration Verification in TIA Portal

Once the LC LED is green, proceed to verify the TIA Portal configuration matches the connected load cell. Open the project, navigate to Devices & Networks → WP231 → Properties → Module parameters, and check the following:

1. Number of connected load cells — set to 1 for a single cell, 2, 3, or 4 for parallel cells of equal rating. Mismatching this field is the single most common cause of the "Downloaded and existing configuration are different" diagnostic.
2. Characteristic value (rated load) — must match the cell's nameplate kg or t rating, e.g., 100.0 kg for a 100 kg cell.
3. Characteristic value per cell — the rated load of an individual cell when multiple cells are connected in parallel (e.g., 4 × 1000 kg under one silo = 1000 kg per cell).
4. Rated output (mV/V) — typically 2.0000 mV/V. Setting this too high reduces sensitivity; too low saturates the ADC at full load.
5. Filter settings — leave the default low-pass filter at the recommended 2 Hz cutoff for static weighing applications; change to 5 to 10 Hz for dynamic / checkweighing applications.

After parameter changes, perform a full Download to device → Hardware and software (not just software). The CPU's diagnostic buffer should clear within 30 seconds, and the SF LED on both CPU and module should go off.

8. Resolving "Downloaded and existing configuration are different"

If the error persists after correcting the wiring and verifying TIA Portal parameters:

1. Open Online & Diagnostics → WP231 → Diagnostic buffer in TIA Portal.
2. Read the entry timestamp and event code. Event code 0x05 with identifier "Configuration deviation" confirms a stored-configuration mismatch.
3. Select Functions → Reset to factory settings from the module's online context menu. This erases the persistent parameter block in the WP231.
4. Re-download the hardware configuration. The WP231 will re-initialize with the TIA Portal parameters and the SF LED should clear.

If the diagnostic buffer still reports a configuration deviation after a factory reset and re-download, the module is suspect and should be replaced. Keep a record of the diagnostic buffer entries (Export → Save as text) before contacting Siemens Industry Online Support with the serial number and firmware version of the WP231.

9. Operating Errors: Checksum Mismatch Recovery

A second, firmware-level error class is documented in the official SIMATIC manual collection: the electronic weighing system reports that the checksum of its firmware (FW) no longer matches the expected value. The entry appears in the diagnostic buffer as Operating error — Checksum error and corresponds to diagnostic interrupt category "Operating errors."

The published remedy path is:

1. Execute the Load factory settings command from the WP231 online diagnostics context in TIA Portal. This restores the parameter block to the as-shipped state.
2. Re-download the TIA Portal hardware configuration so the module boots with the project-defined parameters.
3. If the checksum error reappears after a power cycle, the flash memory of the module has degraded and the module must be returned to SIWAREX Support. The full diagnostic message catalog and event codes for this class are documented in the SIMATIC ET 200SP / SIWAREX manual collection, see Operating errors — technology modules (TIA Siemens Cloud).

10. Verification and Commissioning Tests

Once the LC LED is steady green and the diagnostic buffer is clear, perform these four verification tests before handing the system over to operations:

1. Zero verification — with the empty scale, read the gross weight tag (typically address %IW in the process image). It should be within the zero-tolerance window configured in TIA Portal (default ±0.5% of rated load).
2. Span verification — apply a known calibration weight of at least 50% of rated load. The displayed weight must match the applied mass within the configured accuracy class (typically ±0.05% for a precision cell).
3. Repeatability — apply and remove the same calibration weight five times. The reading should agree to within the resolution of the display (typically 0.1 kg for a 100 kg cell).
4. Diagnostic interrupt test — deliberately disconnect the load cell and confirm the CPU receives a diagnostic interrupt (OB82) and the LC LED goes red. Reconnect and confirm the interrupt clears within one update cycle. This validates the wiring of the diagnostic interrupt path, not just the load cell.

11. Pinout Reference Diagram

12. Troubleshooting Matrix

13. FAQ